I suck, but you're worse
Join Date: Jun 2005
Bikes: Motobecane Fantom Uno-Got rid of the rest when I moved to LA:(
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A couple of things can cause this. First check the things Mike T suggests, could be that there is something in the cassette/freewheel or there is a tight link.-Since you haven't ridden it in a while the chain could very well have a stiff link.
More Likely the chain is worn out_or conversely brand new. You can check this easily by seeing if there are some gears you get this skipping and others where you do not. Usually with an old chain you will get the skipping on gears you dont use as much.
Explaination: As you ride the bushings on the chain(the little rollers between the link plates) wear down on the pins that hold the plates together. This causes the rollers to gain play, in turn causing them to not engage the gear teeth properly(some people call this chain stretch-misnomer). Usually this will also have a wearing effect on the most used rear cogs too. When you shift into a cog you dont use as much it is not worn in the same way as the ones that you do use thereby causing the intermittent skipping.
The solution- You cannot just get a new chain as they wont mesh with the worn cassette/freewheel cogs and you will still have skipping. You need to get both the chain and the cassette/freewheel to solve this problem. These parts of a bike are both considered consumable parts and need to be replaced every 1000 miles or so depending on your weight, quality of thee parts and how hard you ride.